1. Field of the Invention
The present invention generally relates to a semiconductor device and a method of producing the same and, more particularly, to a semiconductor device of a surface mount package type and a method of producing the same.
2. Description of the Related Art
Generally, a BGA (Ball Grid Array) type semiconductor device is well known as a semiconductor device of the surface mount package type. The BGA-type semiconductor device has ball-like protrusion electrodes (bumps) as external connecting terminals, and the protrusion electrodes are bonded to a printed circuit board so that the semiconductor device is mounted onto the printed circuit board.
In recent years, there has been an increasing demand for highly reliable electronic equipment onto which semiconductor devices are mounted, and, hence, high reliability is also expected when a semiconductor device is mounted onto a printed circuit board.
FIG. 1 shows a conventional semiconductor device of a surface mount package type. A BGA-type semiconductor device 1 comprises a semiconductor chip 2, a wiring board 3 (hereinafter referred to as "substrate 3"), protrusion electrodes 4 (hereinafter referred to as "balls 4"), and a mold resin 5. The substrate 3 comprises insulating resin tape 6 made of polyimide (hereinafter referred to as "PI tape") and a wiring layer 7. The semiconductor chip 2 is mounted on the upper surface of the substrate 3 with a bonding member 10.
Ball attachment holes 8 are formed in ball attachment positions in the PI tape 6 of the substrate 3. The wiring layer 7 is made of copper foil formed into a predetermined pattern. Metal wires 9 are bonded between the wiring layer 7 and the semiconductor chip 2, and are connected to the balls 4 through the ball attachment holes 8. In this manner, the semiconductor chip 2 is electrically connected to the balls 4 via the metal wires 9 and the wiring layer 7.
The balls 4 are bumps which function as external connecting terminals, and are formed by soldering. The balls 4 are bonded to the wiring layer 7 through the ball attachment holes 8 formed in the PI tape 6. The balls 4 are also disposed on the mounting surface of the substrate 3 (on the bottom surface in FIG. 1) in an area array so as to accommodate the high-density semiconductor chip 2 and the small semiconductor device 1.
The mold resin 5 is formed on the upper surface of the substrate 3, on which the semiconductor chip 2 is mounted, so as to protect the semiconductor chip 2, the wiring layer 7, and the metal wires 9.
The semiconductor device 1 is surface-mounted on a printed circuit board 11. More specifically, the balls 4 are positioned with electrodes 12 formed on the printed circuit board 11, and the semiconductor device 1 is then placed on the printed circuit board 11. The balls 4 are bonded to the electrode 12 by reflow soldering, so that the semiconductor device 1 is mounted on the printed circuit board 11.
The semiconductor chip 2 in the semiconductor device 1 generates heat when operated. The temperature of the semiconductor chip 2 rises during when operated, and drops when stopped. By the heat, the semiconductor device 1 thermally expands.
However, since the thermal expansion coefficients of the semiconductor device 1 and the printed circuit board 11 are different, a thermal expansion difference occurs between the semiconductor device 1 and the printed circuit board 11, thereby causing stress in the bonding position between the semiconductor device 1 and the printed circuit board 11. Such stress might results in removal of the balls 4 from the electrodes 12.
To solve this problem, a semiconductor device 20 shown in FIG. 2 has been developed. In FIG. 2, the same components as in the semiconductor device 1 shown in FIG. 1 are indicated by the same reference numerals.
The semiconductor device 20 is characterized by a buffer member 21 interposed between the semiconductor chip 2 and the substrate 3. The buffer member 21 is made of elastomer (a low elastic modulus material), and is elastically deformed. The substrate 3 is disposed on the bottom surface (the surface facing the printed circuit board 11) of the buffer member 21.
The semiconductor chip 2 has a face-down structure, and the semiconductor chip 2 and the substrate 3, between which the buffer member 21 is interposed, are electrically connected by the metal wires 9. A potting resin 22 formed by potting seals the semiconductor chip 2.
Transfer molding cannot be performed on the structure having the buffer member 21 that can be elastically deformed. For this reason, the potting resin 22 seals the semiconductor chip 2 and the metal wires 9.
The buffer member 21 interposed between the semiconductor chip 2 and the substrate 3 takes up a thermal expansion difference between the semiconductor device 20 and the printed circuit board 11. Thus, stress is prevented between the semiconductor chip 2 and the substrate 3, and the bonding reliability (packaging reliability) between the balls 4 and the electrodes 12 can be improved.
With the semiconductor device 20, however, there is a problem that the buffer member 21 interposed between the semiconductor chip 2 and the substrate 3 adds to the number of components and complicates the production procedures, resulting in high production costs.
Another problem is that the semiconductor device 20 becomes taller than the semiconductor device 1 by the height of the buffer member 21 between the semiconductor chip 2 and the substrate 3.
The semiconductor chip 2 attached to the buffer member 21 can be sealed only by the potting resin 22, and a fillet-like concave portion 23 is inevitably formed on the boundary of the potting resin 22 and the upper surface of the semiconductor chip 2. Because of the concave portion 23, the upper surface of the semiconductor device 20 formed by the semiconductor chip 2 and the potting resin 22 is not flat, resulting in another problem that desirable chucking cannot performed with the semiconductor device 20.
If the semiconductor device 20 is transported using a vacuum chuck, it is necessary to chuck the upper surface of the semiconductor device 20 having the balls 4 formed on the mounting surface (the bottom surface) of the substrate 3. Especially for the small chip size package shown in FIG. 2, it is necessary to vacuum chuck the entire upper surface of the semiconductor device 20 (i.e., the entire upper surface formed by the semiconductor chip 2 and the potting resin 22).
With the potting resin 22, however, the upper surface of the semiconductor device 20 is not flat due to the fillet-like concave portion 23 on the upper edge. Furthermore, the entire surface of the vacuum chuck needs to be in contact with the upper surface of the semiconductor device 20, but air entering through the concave portion 23 lowers the degree of vacuum and hinders secure chucking.